Photoconductive target of an image tube

ABSTRACT

A target for an image pickup tube comprising a lighttransmitting conductive layer supported on a light-transmitting substrate and a photoconductive layer with rectifying contact at least at one of its sides; in which the photoconductive layer has a portion 1,000 A or thicker which comprises a multiplicity of thin films of two or more different materials with different photoconductive characteristics, each having a thickness of not more than 100 A, the thin films of the different materials being laid alternately one on another.

United States Patent [191 Maruyama et a1.

[451 Mar. 26, 1974 PHOTOCONDUCTIVE TARGET OF AN Primary Examiner-MartinH. Edlow Attorney, Agent, or Firm-Craig and Antonelli [5 7] ABSTRACT Atarget for an image pickup tube comprising a lighttransmittingconductive layer supported on a light- IMAGE TUBE [75] Inventors: EiichiMaruyama, Kodaira; Tadaaki Hirai, Koganei; Kiyohisa Inao, l-lachioji;Naohiro Goto, Machida, all of Japan [73] Assignees: Hitachi, Ltd.;Nippon Hoso Kyokai, both of Tokyo, Japan [22] Filed: Apr. 4, 1973 [21]Appl. No.: 347,663

[30] Foreign Application Priority Data Apr. 7, 1972 Japan 47-34359 [52]US. Cl... 317/235 R, 317/235 N, 317/235 NA, 317/235 AC, 317/241 [51]Int. Cl. H011 15/00; 250/213 [58] Field of Search 317/235 N, 235 AC,241, 317/234 S, 235 NA; 313/66 [5 6] References Cited UNITED STATESPATENTS 3,439,240 4/1969 Geib, Jr. 317/241 transmitting substrate and aphotoconductive layer with rectifying contact at least at one of itssides; in which the photoconductive layer has a portion 1,000 A orthicker which comprises a multiplicity of thin films of two or moredifferent materials with different photoconductive characteristics, eachhaving a thickness of not more than 100 A, the thin films of thedifferent materials being laid alternately one on another,

6 Claims, 6 Drawing Figures PATENTEUMAMG m4 3,8001% IO E E "v E E 8-Aszsea 7) 0 2 Z 6- E 5: '3 j 2- d w 11: m 1 A 400 500 600 700 800 400 $0600 700 800 WAVE LENGTH (mp) WAVE LENGTH (m l l6 8 FIG. ,4 l3

v l9 a w n V IL 9 n IT PHOTOCONDUCTIVE TARGET OF AN IMAGE TUBE Thepresent invention relates to a target of animage pickup tube or more inparticular to a target of an image pickup tube of vidicon type utilizingthe rectifying contact of a photoconductive semiconductor.

The materials of the target for the image pickup tube now in commercialuse include photoconductive semiconductors such as Sb S PbO and Si. Atarget of Sb S is characterized by an ohmic contact, while those made ofPbO or Si generally have a rectifying contact or is of a PN junctiontype. The target with the rectifying contact or PN junction has manyadvantages including the fact that they allow less dark current, ishigher in sensitivity and greater in response speed than the target withan ohmic contact. The manufacture of an image pickup tube with a PNjunction of single crystal such as a vidicon with Si target requireshighly complex processes, and also it is very difficult to remove animperfection of a picture attributable to the defects of a crystal bulkor unsatisfactory processes. A target of a thin film with a rectifyingcontact, by comparison, is manufactured by comparatively simpleprocesses, but it is not an easy matter to achieve a rectifying contactwhich is used successfully for a vidicon target, the known materialssuitable for such a purpose being limited to PhD and few othersincluding selenium and its compounds.

Further, in the conventional co-evaporation method of forming anevaporated thin film of a compound semiconductor with a desirablesensitive spectral region by controlling its composition, a plurality ofdifferent component elements are deposited simultaneously from aplurality of sources of evaporation. In such a method, however, theplurality of sources are located at different positions relative to thesubstrate and therefore the composition of the resulting layer on thesubstrate is not necessarily uniform at every point on the substrate.

For the above mentioned reasons, it is difficult in the conventionalmethods to obtain a thin film with a rectifying contact by evaporationwhich fully meet, all the requirements of an image pickup tube includinga superior dark current characteristic, spectral sensitivitycharacteristic and after image.

The inventors have discovered that the alternate deposition of amultiplicity of thin films of different substances one on another makesit possible to obtain a photoconductive layer with a photoconductivecharacteristic quite similar to that of a material resulting from theuniform mixing of such substances. Generally, when a plurality of thinfilms with different photoconductive characteristics are laid one onanother, the resulting characteristic is the sum of the differentphotoconductive characteristics involved. If, however, the thickness ofeach component thin film is sufficiently small and the laminatedstructure consists of a multiplicity of such thin films depositedalternately one on another, the resulting photoconductive characteristicis intermediate with respect to those of the component thin films.

Accordingly, it is an object of the present invention to obviate thedisadvantages of the conventional target of an image pickup tube and toprovide a novel target of an image pickup tube which is capable ofcontrolling the spectral sensitivity characteristic of the image pickuptube considerably without adversely affecting its characteristics ofdark current and after image.

In order to achieve the above mentioned object, the target of the imagepickup tube according to the present inventioncomprises alight-transmitting substrate, a light-transmitting conductive layer anda photoconductive layer deposited on the light-transmitting conductivelayer and including a part 1,000 A or thicker consisting of at least twothin films of different photo-conductive characteristics alternatelylaid one on another, each of the thin films being A or thinner saidphoto-conductive layer having at least one rectifying contact on itssurface.

The above and other objects, features and advantages will be madeapparent by the detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a diagram showing a section of a target for the image pickuptube according to the present invention;

FIG. 2 is a diagram showing the spectral sensitivity of a conventionalphoto-conductive double layer consisting of Se and AS253;

FIG. 3 is a diagram showing the spectral sensitivity ofa layerconsisting of Se and AS253 used for the target for the image pickup tubeaccording to the present invention; and

FIGS. 4 to 6 are diagrams showing sections of embodiments of the presentinvention.

EMBODIMENT 1 An embodiment of the invention is illustrated in F IG. 1,in which reference numeral 1 shows a glass substrate, numeral 2 alighttransmitting conductive layer, and numeral 3 a photo-conductivelayer. The photoconductive layer 3 of about 3 microns thick is made bydepositing by evaporation a multiplicity of selnium films each about 20A and As Se films each about 7 A alternately one on another in a vacuumof 5 X 10 Torr. A rectifying contact is formed between thelighttransmitting conductive layer 2 and the photoconductive layer 3. Aphoto-conductive layer 3 consisting of only a selenium film has asuperior rectifying contact but has the disadvantage of insufficientsensitivity to light of long wavelength. When the photo-conductive layer3 is formed of only As se by contrast, a high sensitivity to red lightis obtained but it is impossible to form a superior rectifying contact.

In case the photo-conductive layer 3 were made of comparatively thick Seand As Se films of, say, 5,000 A each, two peaks of spectral sensitivitycorresponding to those of the individual component films of Se and As Serespectively would be observed as shown in FIG. 2. The photo-conductivelayer 3 according to the present invention, by contrast, which comprisesa multiplicity of comparatively thin Se and As Se alternate films of,say, 50 A in thickness each having its peak of spectral sensitivityintermediate with respect to the two hypothetical peaks which otherwisemight occur separately for the two component film elements. In thelatter case, it is considered that the photo-conductive layer 3 hassimilar characteristics as if Se and As se are mixed uniformly. Suchsimilarity is achieved only when the thickness of each film of Se and AsSe is approximately 100 A or less.

Further, it is possible by employing the construction of thephoto-conductive layer according to this embodiment to achieve aphoto-conductive layer superior in dark current characteristic, afterimage characteristic and sensitivity to long wavelength light as shownin Table 1 without adversely affecting the rectifying characteristic.

TABLE 1 Characteristics of Target for Image Pickup Tube According toEmbodiment 1 Furthermore, although in the above embodiment the differenttypes of film may be employed, more than three different types of filmsmay be deposited one on another to obtain the photo-conductive layeraccording to the invention by using as many evaporation sources. Also,it is possible to achieve any desired mixing ratio by varying thethickness of each film accordingly.

For the practical purpose of controlling the spectral sensitivity of thephoto-conductive layer the above mentioned multi-layer construction isnot necessarily required all over its total thickness. Most of theenergy of light entering the photo-conductive layer is absorbed at itsportion within the depth of several thousands of A from the surfacethereof. Therefore, the portion of the layer as thick as l,000 A or morefrom the surface has a controlling effect upon the spectral sensitivityof the photo-conductive layer.

If the photo-conductive layer with the above mentioned construction isto be used for a target of the image pickup tube, its dark currentcharacteristic, after image characteristic, lag characteristic as wellas the spectral sensitivity are required to be maintained at asatisfactory level for the image pickup tube. It is already mentionedthat a desirable construction for the target of an image pickup tube isof the rectifying contact type. Since the characteristic of the targetof the rectifying contact type largely depends upon the shape oftherectifying barrier, the portion of the photoconductive multi-layer forcontrolling the spectral sensitivity must be so selected as not toadversely affect the other characteristics of the photo-conductivelayer. To achieve maximum utilization of incident light, it isrecommended that the multi-layer portion of the photoconductive layer belocated as near to the plane of incidence as possible so far as theeffect of the rectifying barrier is not reduced.

The photo-conductive layer used as a target for an image pickup tubegenerally has the thickness of 2 to 20 microns, and it is possible, byproviding a portion of the photo-conductive layer as thick as 1,000 Afor controlling the spectral sensitivity of the photo-conductive layer,to limit the function of such portion to the controlling of the spectralsensitivity without any substantial effect upon the lag, after image anddark current characteristics of the photo-conductive layer.

EMBODIMENT 2 Referring to FIG. 4 showing another embodiment of theinvention, reference numeral 4 denotes a glass substrate, numeral 5 alight-transmitting conductive layer,

and numeral 6 a photo-conductive layer approximately 2 microns thickconsisting of a multiplicity of selenium films each about 5 A thick andCdSe films each about 20 A thick alternately deposited one on another.Reference numeral 7 shows a CdTe film with a thickness of approximately1,000 A. In the embodiment under consideration, a rectifying contact isinterposed between the photo-conductive layer 6 and the CdTe film 7. Aphoto-conductive layer consisting of only selenium films develops nosufficient sensitivity to light of long wavelengths, while one with onlyCdSe films does not satisfy the requirements for rectifying contact. Theemployment of a multi-layer as represented by the photoconductive layer6 consisting of alternately deposited multiplicity of selenium and CdSefilms makes it possible to obtain a target superior both in rectifyingcontact and in sensitivity to long wavelength as is apparent from Table2.

TABLE 2 Characteristics of Target for Image Pickup Tube According toEmbodiment 2 dark target current wavelength construcat target at peaklag (after after tion voltage spectral 3 fields) image of 50 Vsensitivity selenium only I nA 400 my. 6% long CdSe only 35 nA 650 mp.30% long this invention 2 nA 600 mp. 15% short In this embodiment, CdTemay be replaced by ZnS, CdS, ZnSe, CdSe or a mixture of any ones of themwhich has a property similar to that of CdTe.

EMBODIMENT 3 A third embodiment of the invention is shown in FIG. 5. Inthe figure, reference numeral 8 shows a glass substrate, numeral 9 alight-transmitting conductive layer, the numeral 10 a film of MnF2 200Athick. The interposition of the insulating material MnF between thelight-transmitting conductive layer 9 and the selenium film 11 permitsthe reverse breakdown voltage of the rectifying contact between thelight-transmitting conductive layer 9 and the selenium film 11 to beincreased. The purpose of increasing the reverse breakdown voltage ofthe rectifying contact is also achieved by the interposition of PbF CaFMgF A1 0 SiO, ZnS, AS283, or the like insulating material instead of MnFIncidentally, the thickness of the insulating film may be in the rangefrom 10 to 1,000 A.

The selenium film 11 which is approximately 3 microns in thickness has acentral portion 12 approximately 1,000 A thick including a multiplicityof selenium films each about 20 A and tellurium films each about 12 Aalternately laid one on another. There is provided on the selenium film11 an Sb S film 13 about 10,000 A thick to improve the landing of thescanning electron beams. Although the Sb S film 13 may be deposited byevaporation in a vacuum of 1 X 10 Torr. or thereabouts, a porous Sb Sfilm deposited by evaporation in an argon gas of about 5 X 10 Torr. ismore effective for the purpose of effective landing of electron beams.However, since an Sb S film which is porous through its whole thicknessis too high in resistance, resulting in inferior characteristics, it isdesirable that a porous film of Sb s or As se be laid on a solid film ofSb S deposited by vacuum evaporation to obtain an integrated Sb S film.In the embodiment under consideration, the central portion 12 in theform of a multi-layer is such that an improved sensitivity to red lightis obtained as shown in Table 3 without adversely affecting therectifying contact formed of the light-transmitting conductive layer 9,insulating film and the selenium film 11.

TABLE 3 Characteristics of Target for Image Pickup Tube According toEmbodiment 3 dark target current wavelength construcat target at peaklag (after after tion voltage spectral 3 fields) image of 50 Vsensitivity selenium only I nA 400 my. 67r long Te only large thisinvention l nA 600 my. 7% short EMBODIMENT 4 A fourth embodiment of theinvention is illustrated in FIG. 6. Reference numeral 14 shows a glasssubstrate, numeral 15 a light-transmitting conductive layer, numeral 16a ZnSe film approximately 500 A thick, and numeral 17 a photo-conductivelayer about 3 microns thick which comprises a multiplicity of seleniumfilms each about 40 A and arsenic films each about 5 A alternately laidone on another. The superior rectifying contact between the ZnSe film 16and photoconductive layer 17 is also achieved by using a ZnS or CdSefilm of the same thickness in place of the ZnSe film. In thisembodiment, too, an insulating film may be inserted on the side of therectifying contact of the photo-conductive layer in order to improve thebreakdown voltage of the rectifying contact in the reverse direction inthe preceding embodiment. The photoconductive layer 17 is provided witha central multilayer portion 18 about 2,000 A thick comprising amultiplicity of selenium films each about A, arsenic films each about 5A and tellurium films each about 30 A which are deposited alternatelyone on another.

An Sb- S film 19 about 1,000 A thick is provided on the photowonductivelayer 17 to improve the landing of the scanning electron beams.According to the present embodiment, the presence of the tellurium filmsin the layer 113 permits the sensitivity to red light to be improved asshown in Table 4 without adversely affecting the advantage of therectifying contact.

TABLE 4-Characteristics of Target for Image Pickup Tube According toEmbodiment 4 It will be understood from the above explanation that, in atarget for an image pickup tube with a rectifying contact, the provisionof a multi-layer consisting of a multiplicity of thin films of two ormore different photo-conductive types alternately deposited one onanother makes it possible to control more widely the spectralsensitivity of the target without adversely affecting the othercharacteristics thereof. For this reason, the present invention isapplied with great advantages to the construction of a target for acolor television image pickup tube or the like whose requrements forspectral sensitivity are very severe.

What we claim is:

1. A photo-conductive target for an image pickup tube comprising alight-transmitting substrate, a lighttransmitting conductive layerdeposited on said lighttransmitting substrate and a photo-conductivelayer deposited on said light-transmitting conductive layer, saidphoto-conductive layer containing selenium and including a portion notless than 1,000 A thick, said portion of said photo-conductive layerconsisting of a multiplicity of thin films of at least two differentmaterials with different photo-conductive characteristics, each havingthe thickness of A or less, said thin films being laid alternately oneon another.

2. A photo-conductive target for an image pickup tube according to claim1, further comprising one semiconductor layer selected from the groupconsisting of ZnS, CdS, ZnSe, CdSe, CdTe and a mixture thereof depositedon said photo-conductive layer, a rectifying contact being formed bysaid semiconductor layer and said photo-conductive layer.

3. A photo-conductive target for an image pickup tube according to claim1, in which said lighttransmitting layer and said photo-conductive layerconstitute a rectifying contact.

4. A photo-conductive target for an image pickup tube according to claim2, in which an insulating thin film with the thickness from 10 A to1,000 A and comprising one selected from the group consisting of ZnS,PbF MnF CaF MgF A1 0 SiO and AS283 is provided adjacent to that side ofsaid photo-conductive layer which has said rectifying contact.

5. A photo-conductive target for an image pickup tube according to claim3, in which an insulating thin film with the thickness from 10 A to1,000 A and comprising one selected from the group consisting of ZnS,

deposited on a vacuum-evaporated Sb S film.

2. A photo-conductive target for an image pickup tube according to claim1, further comprising one semiconductor layer selected from the groupconsisting of ZNS, CdS, ZnSe, CdSe, CdTe and a mixture thereof depositedon said photo-conductive layer, a rectifying contact being formed bysaid semiconductor layer and said photo-conductive layer.
 3. Aphoto-conductive target for an image pickup tube according to claim 1,in which said light-transmitting layer and said photo-conductive layerconstitute a rectifying contact.
 4. A photo-conductive target for animage pickup tube according to claim 2, in which an insulating thin filmwith the thickness from 10 A to 1,000 A and comprising one selected fromthe group consisting of ZnS, PbF2, MnF2, CaF2, MgF2, Al2O3, SiO andAs2S3 is provided adjacent to that side of said photo-conductive layerwhich has said rectifying contact.
 5. A photo-conductive target for animage pickup tube according to claim 3, in which an insulating thin filmwith the thickness from 10 A to 1,000 A and comprising one selected fromthe group consisting of ZnS, PbF2, MnF2, CaF2, MgF2, Al2O3, SiO andAs2S3 is provided adjacent to that side of said photo-conductive layerwhich has said rectifying contact.
 6. A photo-conductive target for animage pickup tube according to claim 1, in which the surface of saidphoto-conductive layer to be scanned by an electron beam is covered withone selected from the group consisting of a vacuum-evaporated Sb2S3film, a porous Sb2S3 film, a combination of a vacuum-evaporated Sb2S3and a porous Sb2S3 film, and a porous As2Se3 film deposited on avacuum-evaporated Sb2S3 film.